Field of the Invention
The present invention relates to an electron microscope and also to a method of operating it.
Description of Related Art
Generally, electrons emitted from a field-emission electron gun contain a varying portion of several percent for the following reason. Gases and ions are adsorbed onto the surface of the emitter and migrate, varying the work function of the metal surface. Also, collision of ions and so on varies the geometry of the metal surface. Therefore, where a field-emission electron gun is used in a scanning transmission electron microscope (STEM), a detector for noise cancellation is mounted in the electron optical column to detect nearby electrons that form a probe. The signal emitted from the sample is divided by the resulting detection signal, whereby emission noise on the image is eliminated. This noise canceling technique is disclosed, for example, in JP-A-5-307942.
FIG. 15 shows the configuration of a scanning transmission electron microscope (STEM) 101 having a general noise cancellation function. This electron microscope 101 of FIG. 15 has an electron optical column 110 in which various components including a cold field-emission electron gun (CFEG) 111, a noise canceling aperture 112, a lens 113a, scan coils 113b, another lens 114, a detector 115, a preamplifier circuit 120, and an amplifier circuit 130 are housed.
The electron beam emitted from the CFEG 111 is partially cut off by the noise canceling aperture 112 and then converged onto a sample A by the lens 113a. The converged beam is scanned over the sample A by the scan coils 113b. The electron beam transmitted through the sample A passes through the lens 114, and a part of the beam is detected by the detector 115.
An image signal detected by the detector 115 is the product of an emission current I1 impinging on the sample A and the brightness component S of the sample A, i.e., I1×S. The emission current I1 impinging on the sample A and the emission current I2 detected by the noise canceling aperture 112 have a proportional relationship, i.e., I1=n×I2. An offset is added to the image signal (I1×S) and the resulting signal is amplified by a factor of Gp by the preamplifier circuit 120. The amplified signal is further amplified by a factor of Ga by the amplifier circuit 130.
On the other hand, the emission current I2 detected by the noise canceling aperture 112 is amplified by a factor of Gn by a noise detection circuit 140. When the noise cancellation function is not used, the output signal of the amplifier circuit 130 bypasses a noise canceling circuit 150 and is arithmetically processed in a given manner by an arithmetic section (CPU) 160 and then sent to a personal computer (PC) 102. As a result, an STEM image of the sample A is displayed on a display unit for use with the PC 102.
When the noise cancellation function is used, the offset component added by the preamplifier circuit 120 is subtracted from the output signal of the amplifier circuit 130 by the noise canceling circuit 150. Then, the resulting signal is divided by the output signal of the noise detection circuit 140. Consequently, the emission noise contained in the image signal is removed. The image signal free of the emission noise is arithmetically processed in a given manner by the arithmetic section (CPU) 160 and sent to the personal computer (PC) 102. An STEM image of the sample A free of the emission noise is displayed on the display unit for use with the PC 102.
FIG. 16 shows a specific example of configuration of signal processing circuitry when the electron microscope 101 is in a mode of operation where the noise cancellation function is not used. As shown in this figure, when the noise cancellation function is not in use, STEM imaging is done fundamentally using only two adjustments, i.e., contrast and brightness. Contrast is a gain added to an image signal for adjusting the brightness. Brightness is a DC voltage applied to cancel out the offset component of the image signal. In the example of FIG. 16, with respect to the image signal S×I1 obtained from the detector 115 by adjusting the contrast, brightness B is added to the image signal S×I1 by an adder 122 in the preamplifier circuit 120 and then amplified by the factor of Gp by an amplifier 124. Therefore, the output signal V11 of the amplifier 124 is given byV11=Gp×(S×I1+B)  (A)
The output signal V11 of the amplifier 124 is amplified by the factor of Ga by an amplifier 132 in the amplifier circuit 130. Thus, from Eq. (A) above, the output signal V12 of the amplifier 132 is given byV12=Ga×Gp×(S×I1+B)  (B)
The output signal V12 of the amplifier 132 is converted from analog to digital form by an analog to digital converter (ADC) 162 in the arithmetic section 160, then averaged or otherwise arithmetically processed, and sent to the PC 102 shown in FIG. 15.
On the other hand, FIG. 17 shows a specific example of configuration of signal processing circuitry when the electron microscope 101 is in a mode of operation where the noise cancellation function is used. As shown in this figure, also when the noise cancellation function is used, the output signal V12 of the amplifier 132 is given by Eq. (B) above. In order to cancel out the brightness B added by the preamplifier circuit 120, an amplifier 151 of the noise canceling circuit 150 adds a gain equal to the product of the gain Gp of the amplifier 124 and the gain Ga of the amplifier 132 to the brightness B. A subtractor 152 subtracts the output of the amplifier 151 from the output signal V12 of the amplifier 132. Accordingly, it is seen from Eq. (B) above that the output signal V13 of the subtractor 152 is given by
                                                                        V                13                            =                            ⁢                                                Ga                  ×                  Gp                  ×                                      (                                                                  S                        ×                        I                        ⁢                                                                                                  ⁢                        1                                            +                      B                                        )                                                  -                                  Ga                  ×                  Gp                  ×                  B                                                                                                        =                            ⁢                              Ga                ×                Gp                ×                S                ×                I                ⁢                                                                  ⁢                1                                                                        (        C        )            
The emission current I2 detected by the noise canceling aperture 112 is converted into a voltage and amplified by the factor of Gn by an amplifier 142 in the noise detection circuit 140. Therefore, the output signal V14 of the amplifier 142 is given byV14=Gn×I2  (D)
The output signal V13 of the subtractor 152 is applied to a numerator input (X) of a divider circuit 154. The output signal V14 of the amplifier 142 is applied to a denominator input (Y) of the divider circuit 154. Accordingly, from Eqs. (C) and (D), the output signal V15 of the divider circuit 154 is given by
                              V          15                =                              X            Y                    =                                                    V                13                                            V                14                                      =                                          Ga                ×                Gp                ×                S                ×                I                ⁢                                                                  ⁢                1                                            Gn                ×                I                ⁢                                                                  ⁢                2                                                                        (        E        )            
In the noise canceling circuit 150, in order to subtract the output signal of the amplifier 151 from the output signal V12 of the amplifier 132 by the subtractor 152, an amplifier 155 adds a gain equal to the product of the gain Gp of the amplifier 124 and the gain Ga of the amplifier 132 to the brightness B. An adder 156 adds the output of the amplifier 155 to the output signal V15 of the divider circuit 154. Therefore, the output signal V16 of the adder 156 is given by
                                                                        V                16                            =                            ⁢                                                                    Ga                    ×                    Gp                    ×                    S                    ×                    I                    ⁢                                                                                  ⁢                    1                                                        Gn                    ×                    I                    ⁢                                                                                  ⁢                    2                                                  +                                  Ga                  ×                  Gp                  ×                  B                                                                                                        =                            ⁢                                                S                  ×                                                            Ga                      ×                      Gp                                        Gn                                    ×                                                            I                      ⁢                                                                                          ⁢                      1                                                              I                      ⁢                                                                                          ⁢                      2                                                                      +                                  Ga                  ×                  Gp                  ×                  B                                                                                        (        F        )            
The output signal V16 of the adder 156 is converted from analog to digital form by the analog to digital converter 162 in the arithmetic section 160, then averaged or otherwise arithmetically processed, and sent to the PC 102 shown in FIG. 15.
Substituting the equation, I1=n×I2, into Eq. (F) results in
                              V          16                =                              S            ×                                          Ga                ×                Gp                            Gn                        ×            n                    +                      Ga            ×            Gp            ×            B                                              (        G        )            
Note that none of the emission currents I1 and I2 containing emission noise are present in the right side of Eq. (G). Consequently, when the noise cancellation function is used, a value proportional to the brightness component S of the sample S to be imaged and observed is obtained in the same way as when there is no emission noise.
In the example of FIG. 17, operations for removing and re-adding brightness and a division operation are performed by analog circuitry. Alternatively, these operations may be carried out by digital arithmetic operations. In this case, measurement and setting of the gain of brightness that is removed and re-added and other adjustments can be made automatically.
The related art noise canceling method described so far has the following problems.
First, where the division is performed with an analog circuit (herein referred to as the analog division method), it is necessary to constitute log (logarithm) circuits and an antilog circuit. FIG. 18 shows one example of configuration of the divider circuit 154 using log circuits 154a, 154b and an antilog circuit 154c. As shown in FIG. 18, the divider circuit 154 performs a division using analog signals by performing a logarithmic conversion by the log circuits 154a, 154b, then subtracting the output signal of the log circuit 154b from the output signal of the log circuit 154a, and performing a logarithmic conversion of the resulting difference by the antilog circuit 154c. 
In the analog division method, it is necessary to constitute the log circuits 154a, 154b and antilog circuit 154c operating in a frequency bandwidth of several MHz corresponding to the speed at which the image signal is detected and so the amount of noise component increases steeply. However, this frequency bandwidth is required for signals of STEM images and, therefore, it is difficult to limit the bandwidth subsequently. The antilog circuit 154c has a high gain, and it is difficult to broaden the frequency bandwidth due to the effects of noise.
On the other hand, where divisions are performed by digital computations (herein referred to as the digital division method), divisions are slower to perform than other types of calculations. As a result, the frequency bandwidth is narrowed.